03-01-2012, 10:38 PM
Onward and upward, at least after I'd waited for the storm to die down a bit.
After removing V1 and V2 I injected a signal into the primary of T1. The output stage voltages look about right and the output is fine, although there is a touch of hum. Tacking a 32u in various places didn't make any real difference so that may have to wait until it's closer to its real configuration. The bandwidth is a bit limited by modern standards - effectively 8kHz, although it sounds fine on a station.
Trying the same ploy with V2 installed was an entirely different matter. The voltages around V2, when switched to Gram, are slightly out - the Anode voltage is a bit high and the Cathode voltage is low, implying perhaps that the valve is a bit weak. However, it isn't the right one although it is a reasonable substitute according to the data. There are two main problems. First, the Volume Control has no effect on the input - see circuit. That's no real problem when using a Signal Generator, but clearly would be with a real Gram. Second, any connection to the Gram input leads to rampant oscillation, at quite a high frequency. I have got long dangly leads kicking about but dressing them in various ways made no difference. Bear in mind that this is without V1. However, as you can see, there's plenty of feedback paths.
That's as far as I've got today. Tomorrow I think it might be best to investigate the first stage as the components in there will affect the feedback.
Alan
After removing V1 and V2 I injected a signal into the primary of T1. The output stage voltages look about right and the output is fine, although there is a touch of hum. Tacking a 32u in various places didn't make any real difference so that may have to wait until it's closer to its real configuration. The bandwidth is a bit limited by modern standards - effectively 8kHz, although it sounds fine on a station.
Trying the same ploy with V2 installed was an entirely different matter. The voltages around V2, when switched to Gram, are slightly out - the Anode voltage is a bit high and the Cathode voltage is low, implying perhaps that the valve is a bit weak. However, it isn't the right one although it is a reasonable substitute according to the data. There are two main problems. First, the Volume Control has no effect on the input - see circuit. That's no real problem when using a Signal Generator, but clearly would be with a real Gram. Second, any connection to the Gram input leads to rampant oscillation, at quite a high frequency. I have got long dangly leads kicking about but dressing them in various ways made no difference. Bear in mind that this is without V1. However, as you can see, there's plenty of feedback paths.
That's as far as I've got today. Tomorrow I think it might be best to investigate the first stage as the components in there will affect the feedback.
Alan
